The field of this disclosure generally relates to electrolyte material for use in thermal batteries. The disclosure also relates to composites of electrodes and electrolytes that contain the electrolyte material, and to batteries that contain the electrolyte material and/or a cathode and/or an anode that contain the electrolyte material.
Thermal batteries tend to have relatively long shelf lives, high energy densities, require relatively low maintenance, and can withstand relatively high temperatures. Thermal batteries also tend to provide a short burst of power over a relatively short period of time. The burst may range from less than a second to an hour or more, with power typically ranging from about a watt or less to kilowatts. Such properties make thermal batteries suitable for military (e.g., batteries for missile guidance systems) and space exploration applications. Thermal batteries may also be used in other applications, such as in electric vehicles.
A typical thermal battery includes an anode, a cathode, an electrolyte-separator containing a solid electrolyte that is non-conductive at ambient temperature, and a pyrotechnic material (e.g., heat pellet, which may contain, for example, Fe—KClO4 powder) that provides a heat source to the battery. When battery operation is desired, an external stimulus is applied to the battery. For example, an electrical current may be applied to the battery to set off an electric match or an electro-active squib, or a mechanical force (e.g., mechanical shock) may be applied to set off a concussion primer. The external stimulus causes the pyrotechnic material to ignite and begin to heat. Heat produced from the pyrotechnic material causes the previously solid electrolyte to melt and become conductive, which allows the battery to provide power for a desired application.
The anodes of thermal batteries are generally formed of an alkali or alkaline earth metal or alloy. A typical anode includes lithium metal or a lithium alloy, such as lithium aluminum, lithium silicon, or lithium boron.
Electrolytes for use with thermal batteries often include a eutectic mixture (i.e., a mixture which melts at a temperature lower than each of the individual components) of lithium chloride and potassium chloride and a binder (such as MgO, fumed silica or kaolin), which assists in containing the electrolyte within the thermal battery assembly upon melting, such as by capillary action, surface tension, or both. With typical thermal battery electrolytes, a binder prevents the electrolyte material from dispersing throughout the battery, which would cause undesired shunts or short circuits in the cell. Unfortunately, the binder materials tend to be relatively resistant to ionic conduction and thus inclusion of the binder may increase the impedance of the battery.
Cathode materials for thermal batteries may vary in accordance with a variety of design parameters and generally include a metal oxide or metal sulfide. By way of example, iron oxide (Fe3O4), iron disulfide (FeS2) or cobalt disulfide (CoS2) are often used as cathode materials.
Thermal batteries are often formed using pellet techniques, such that each of the electrolyte, cathode, and heat source are formed into a wafer. In this case, the respective cell component chemicals are processed into powders and the powders are pressed together to form the wafer (or pellet). Each component may be formed as a discrete part, or the anode and/or cathode may include (i.e., be flooded with) electrolyte material to improve the conductivity of that component. The electrolyte material in the anode and cathode may or may not contain binder material.
A thermal battery may consist of a single series of stacked cells or two or more parallel stacks of the series of stacked cells. The cell stack(s) may be insulated as thoroughly as possible, placed in a container, which may be made of stainless steel, and the container is sealed to form a hermetic seal, such as by welding. Electrical connections may be provided through standard glass to metal seals.
Silver oxide zinc (SOZ) batteries generally comprise an Ag2O or AgO cathode and a Zn anode. The electrolyte commonly used in SOZ batteries is an aqueous solution of KOH or NaOH. SOZ batteries have a high energy density, allow for flexible configurations, have excellent voltage regulations, and have been proven to be safe and reliable. However, SOZ batteries generally only have a two to five year shelf life, and SOZ batteries generally operate at low temperatures, such as between 4° C. and 70° C. Thus, SOZ chemistry has not been used in thermal batteries due, at least in part, to the above deficiencies.
A continuing need exists for reliable lower-temperature thermal battery materials. A continuing need also exists for primary batteries, such as thermal batteries, that incorporate such materials and exhibit such improved performance.